This invention relates to a method for the preparation of a melt for mineral wool production wherein a raw material having a composition adjusted to the desired composition of the melt is introduced at the top of a shaft furnace and is heated therein to form a melt which is discharged at the bottom of the furnace.
In a well known method of the above mentioned type the raw material and coke are introduced at the top of the shaft furnace so as to form alternating layers therein, and oxygen-containing gas, such as air, is introduced at the lower part of the furnace so as to effect a combustion of the coke and to heat the raw material to melting temperature. The shaft furnace is preferably a cupola furnace and such a furnace ordinarily comprises four temperature zones, viz. a melt bath, an oxidation zone, a reduction zone and a preheating zone.
The melt bath is located at the lower end of the cupola furnace and the temperature in this zone is typically 1500-1550.degree. C.
The oxidation zone is located above the melt bath and the lower part of this zone comprises orifices-- the so-called tuyeres-- for introducing oxygen-containing gas which preferably is preheated.
During the movement of the preheated gas up through the oxidation zone the coke is combusted and the gas temperature is raised from about 500.degree. C. to about 2000.degree. C. The melt formed as a result of the heat generation within the oxidation zone flows downwardly to the melt bath which as mentioned above typically has a temperature of 1500.degree.-1550.degree. C.
The extent of the oxidation zone in the vertical direction is determined by the amount of oxygen supplied to the furnace because the reduction zone starts at the level at which the oxygen supplied through the tuyeres has been used for the combustion of coke.
In the reduction zone in which the temperature is between 1000 and 2000.degree. C. the coke reacts with CO.sub.2 formed as a result of the combustion of the coke in the oxidation zone to form CO in an amount which is twice the amount of the consumed CO.sub.2 on volume basis.
This reaction is endothermic and causes 20-25% of the energy generated by the combustion within the oxidation zone to be lost as latent heat in the flue gas when the latter contains 6-10% CO. Although this heat can be recovered in an after-combustion step, it is desirable to avoid the formation of CO, i.e. because an after-combustion step requires relatively high initial investments and because the efficiency of such an after-combustion is relatively low.
The reduction, i.e. the formation of CO, is strongly temperature dependent at a temperature of above 1000.degree. C. Thus, the CO formation is increased with a factor of 10 when the temperature is increased from 1000.degree. to 1200.degree. C. On the other hand, the rate at which CO.sub.2 is converted into CO is so low at temperatures of below 1000.degree. C. that the reduction within the preheating zone located above the reduction zone and wherein the raw materials introduced at the top of the cupola furnace are heated from the temperature of the surroundings to about 1000.degree. C. is insignificant.
W082/00460 discloses a method for the preparation of a melt for mineral wool production wherein lumps of a raw material are introduced into a shaft at its upper end, said shaft having a lower end located in a horizontally extending melt furnace which is supplied with hot gas from a mixing chamber.
Gas from a plasma generator, finely divided waste material, carbon powder and optionally hydrocarbon gas are mixed within the mixing chamber before the mixture thus obtained is introduced into the horizontally extending melt furnace. After having given off part of its heat content to melt the raw materials introduced into the furnace, the hot gas passes up through the shaft and is introduced into an after-combustion zone having means for supplying air thereto. The combustion gas formed is cleaned, recycled and is introduced into the mixing chamber together with the components mentioned above.